This invention relates generally to materials testing, and more particularly to strain responsive devices which detect the presence of intiial crack growth and subsequent propogation of cracks in structural members.
In the art of materials testing, it is highly desirable to detect a crack in the tested material sometime before it reaches a critical length, at which point unstable crack propagation and structural failure may occur. This is particularly true in cases where stress raisers, i.e., notches, holes, or other discontinuities in contour or structure which cause localized stress concentration, are present in the tested material.
Strain responsive devices, such as those which are bonded to the tested material and dependent upon either the severing of an electrically conductive element or the cracking of a pre-machined notch, have been used in the pars with some degree of success to detect such cracks. However, if a crack in the tested material does not initiate or subsequently propagate directly beneath the device used, the electrically conductive element or pre-machined notch may not break, thus negating the efficacy of the device.